1. Field of the Invention
This invention pertains generally to inspecting sheets of transparent material, and more particularly to observing paired sheets of glass or other transparent material for determining the angular relationship or degree of fit or match of the sheets.
2. Description of the Prior Art
In the flat glass field there are many instances where it is highly desirable to determine whether a sheet of glass has a particular predetermined surface configuration or, more specifically, in what areas and to what degree the sheet deviates from a desired configuration. Such information is of particular significance in the automotive glazing field where glass sheets are bent to various curved forms for utilization as side lites, rear windows, roof panels and windshields. In order to fit properly and meet automotive manufacturers rigid quality standards, as well as government-imposed standards for optical clarity, it is necessary that individually fabricated units conform within narrow limits to a standard configuration. This is particularly true in the fabrication of laminated glazing units wherein matching sheets of glass are laminated under heat and pressure to a plastic interlayer member. Such sheets are formed by pressing heat-softened sheets individually into conformity with a mold or by gravity bending sheets in pairs in order to insure that their configurations closely match. Despite the best-efforts through years of experience in bending glass for such units, it is found that some of the bent sheets warp or distort upon subsequent cooling. This is particularly true in the case of present day laminated windshields in which one of the sheets of the pair may have an opaque band painted on the surface around the edge to conceal the means by which the unit is mounted in the vehicle. The band seems to cause uneven cooling and subsequent warping of the sheet. As a result, when paired the bent sheets will have areas that do not conform to one another. In other words, when the sheets are nested one upon the other, there will be gaps between the two sheets in certain areas. This lack of conformity is not visually detectible. When such sheets are subsequently laminated under heat and pressure to the thin plastic interlayer, they bend into conformity, that is, they assume corresponding configurations. However, in doing so undesirable stresses are created in the sheets. The stresses or residual forces may tend to cause delamination or separation of the sheets from the interlayer over a period of time. More importantly, on forcing the sheets into conformity with one another during laminating, some portions of the outer surfaces of the sheets are placed in tension. As is well known, glass has great strength in compression, but does not function well in tension. The outboard surfaces of laminated vehicle windshields are often subjected to impact by flying objects such as stones and gravel thrown up from the road surface by other vehicles. Where the glass surface is not under stress, such impinging objects may merely scratch or pit the surface of the glass, and the windshield will not be severely damaged. However, if the glass is under tension at the point of impact, cracks or stress fractures may radiate from the damaged area, rendering the windshield unfit for further use. Replacement of the windshield is inconvenient and expensive, and the cracked windshields have resulted in numerous complaints from vehicle owners.
The flat or curved tempered glass parts utilized for present day automobile side and rear window glazings must conform to a predetermined configuration in order to properly fit into the opening provided therefore in the vehicle body. As is well known, such glass parts may warp during the heating and subsequent chilling utilized in their fabrication, resulting in randomly produced parts having a configuration falling outside acceptable tolerances. Heretofore, detection of such units was time consuming and unreliable, often resulting in defective units reaching the automobile assembly line before they were discovered.
Determination of the planar alignment of two sheets of glass relative to one another is important in certain glass fabricating operations. Thus, in the production of multiple sheet insulating glass units the spaced sheets of glass must be parallel to minimize distortion of objects viewed through the finished unit. The invention is particularly adapted to determining such parallelism during production and as a means of checking the finished units for compliance with quality standards.